1. Field of the Invention
The present invention relates generally to power converter circuits, and more specifically relates to a ballast control circuit with an integral power factor correction (PFC) circuit.
2. Description of Related Art
Ballasts have been used for many years as part of lighting systems and gas discharge lamps, and in particular for fluorescent lamps. Fluorescent lamps pose a load control problem to the power supply lines that provide lamp power, because the lamp load is non-linear. Current through the lamp is zero until an applied voltage reaches a starting value, at which point the lamp begins to conduct. As the lamp begins to conduct, the ballast ensures that the current drawn by the lamp does not increase rapidly, thereby preventing damage and other operational problems.
A type of electronic ballast typically provided includes a rectifier to change the alternating current (AC) supplied by a power line to direct current (DC). The output of the rectifier is typically connected to an inverter to change the direct current into a high frequency AC signal, typically in the range of 25-60 kHz. The high frequency inverter output to power the lamp permits the use of inductors with much smaller ratings than would otherwise be possible, and thereby reduces the size and cost of the electronic ballast.
Often, a power factor correction circuit is inserted between the rectifier and the inverter to adjust the power factor of the lamp circuit. Ideally, the load in an AC circuit should be equivalent to pure resistance to obtain the most efficient power delivery for the circuit. The power factor correction circuit is typically a switched circuit that transfers stored energy between storage capacitors and the circuit load. The typical power inverter circuit also employs switching schemes to produce high frequency AC signal output from the low frequency DC input. Switching within the power factor correction circuit and the rectifier circuit can be accomplished with a digital controller.
By controlling the switching in the power inverter circuit, operating parameters of the lamp such as starting, light level regulation and dimming can be reliably controlled. In addition, lamp operating parameters can be observed to provide feedback to the controller for detection of lamp faults and proper operational ranges.
A conventional electronic ballast circuit is shown diagrammatically in FIG. 1A. A power factor correction (PFC) circuit 16 accepts a line input and provides regulated power to an output stage 18. PFC circuit 16 provides a regulated DC bus voltage to output stage 18. Output stage 18 provides appropriate control for powering lamp 26. Output stage 18 includes the components and operational ability for preheating, igniting and regulating power to lamp 26.
PFC circuit 16 is typically realized as a boost-type converter that requires a high voltage switch, an inductor, a diode, a high voltage DC bus capacitor and an associated control circuit to produce the desired power signals with the components provided. Output stage 18 is typically realized with a half-bridge driven resonant load to provide appropriate power to lamp 26. Output stage 18 typically requires two high voltage switches, a resonant inductor, a resonant capacitor, a DC-blocking capacitor and an associated control circuit for regulating circuit resonance and power delivery. A representative circuit diagram of such a conventional circuit is illustrated in FIG. 1B.
In the conventional configuration shown in FIG. 1B, switch M1 constitutes one of the switches of the half-bridge output stage. Switch M1 is connected to a DC bus capacitor Cbus at a single mode. The PFC circuit components Lpfc, Mpfc and Dpfc are operated to charge Cbus during an initial stage, such as a power on state. Upon being charged, bus capacitor Cbus supplies power to half-bridge resonant output stage 18 for the remainder of the operation of the circuit. By supplying power to output stage 18, bus capacitor Cbus is rated for high capacitance and high voltage operation, thereby increasing the cost and size of the electronic ballast circuit. In addition, switches M1, M2 are also rated for high voltage operation, and therefore have increased cost and size as well.
Another application for the type of circuit described in FIG. 1A is for use related to power converters. The range of power converter applications include AC to DC power converters and DC to DC power converters. A conventional AC to DC power converter is illustrated in FIG. 1C. The configuration of the DC converter shown in FIG. 1C is similar to the electronic ballast circuit illustrated in FIG. 1B, but having a different load configuration. The DC converter and FIG. 1C has a power factor correction circuit composed of a high voltage switch Mpfc, an inductor Lpfc, a diode Dpfc, a high voltage DC bus capacitor Cbus and an associated PFC control circuit (not shown). The half-bridge driven resonant load includes two high voltage switches M1, M2, a resonant conductor Lres, a resonant capacitor Cres, a DC blocking capacitor Cblk and an associated control circuit (not shown) to control power delivered to transformer T1, and subsequently to load resistor RL. As with the configuration shown in FIG. 1B, bus capacitor Cbus shown in FIG. 1C is charged by operation of PFC components Lpfc, Mpfc and Dpfc. Bus capacitor Cbus then supplies all power transferred to the half-bridge resonant output stage for the remainder of the operation of the DC converter. Accordingly, bus capacitor Cbus is rated for high capacitance and high voltage operation, resulting in larger and more expensive components. Similarly, the power controlled by switching switches M1, M2 is supplied to the resonant components and output stage of the DC converter. Switches M1, M2 are therefore rated for high voltage operation, resulting again in larger and more expensive components.
The present invention breaks the connection between the line input and the bus capacitor conventionally made in prior converters and ballasts. According to this novel approach, the bus capacitor is accessible to the line input through a half-bridge and a resonant output stage connected in series. By separating the bus capacitor from the input stage, power distribution in the circuit can be achieved with a more balanced operation. In this configuration, the bus capacitor can have a very high ripple voltage, permitting the use of a non-electrolytic capacitor. By using less expensive and more reliable types of capacitors, the overall circuit becomes more reliable, less expensive, and easier to maintain.
According to the present invention, two half-bridges are used with a resonant output stage, with a bus capacitor coupled to one of the half-bridges. The half-bridges, composed of lower voltage switches, are operated to supply power from a rectified line input in conjunction with a DC bus capacitor. The bus capacitor supplies power for only a portion of the input line voltage cycle, rather than for the entire operational time of the circuit.
Accordingly, the two half-bridge circuit configurations achieves bi-directional power flow through the load. Switches in the two half-bridges are controlled to draw a sinusoidal current from the line input to achieve a high power factor. Control of the switches in conjunction with the bus capacitor permits a charge to be placed on the bus capacitor in cycles, thereby providing constant power to the load.
According to an embodiment of the present invention, an input line side half-bridge, composed of two switches, operates to turn one switch on and off to obtain a sinusoidal current from the line input to supply current to the load. The second switch in the first half-bridge is turned on and off with an opposite duty cycle from that of the first switch to provide a recirculation path for bi-directional current flow. In the second half-bridge, one switch is operated to supply current to the load from the bus capacitor to maintain constant power delivered to the load. The other switch in the second half-bridge operates to provide a recirculation path to maintain bi-directional current flow in the circuit, while controlling the charging of the bus capacitor. In the second half-bridge, the first switch operates as a diode when it is turned off to assist in charging the bus capacitor under the control of the second switch.
According to another embodiment of the present invention, the switches in each half-bridge are switched with equal, complementary duty cycles. All switches are switched at substantially the same time, with the frequency of switch operation varying to adjust input current and output voltage.